The reason is that Mint is essentially a Desktop addition to Ubuntu. As server edition does not come with desktops Mint just will not add anything. GUI-tools for maintaining a server are all based on remote and thus you can use pretty much any machine to maintain a Linux server, even a M$ box.
Besides there is not enough resource to maintain more spins

Server versions DO NOT have desktops, therefore there would be no difference to the ubuntu server version already available. Please search before posting, this has already been suggested and rejected any times.